1. Field of the Invention
The present invention relates to an optical sheet and optimizing method thereof, and more particularly, to an optical sheet and optimizing method thereof capable of maintaining contrast throughout a wide viewing angle.
2. Description of the Related Art
Generally, a cathode ray tube (CRT) has been most widely used as an image display devices for displaying images on a screen. However, the CRT is inconvenient since it is large and heavy for the amount of display area provided. Thus, a CRT can only be used in places where both the size and weight of the CRT can be accommodated.
A flat panel display has a large display area while being both thin and light. Thus, a flat panel display can be installed in more places than a CRT. Accordingly, flat panel displays are replacing CRTs. Liquid Crystal Display (LCDs) are especially replacing CRTs since LCDs have excellent resolution compared with other flat panel displays and a response speed that is as fast as the CRT in displaying a moving picture.
The operating principle of the LCD is based on the optical anisotropy and polarization property of liquid crystal molecules. Since a liquid crystal molecule is a thin and long structure, it is possible to control the alignment direction of a string of liquid crystal molecules by applying an electric field across them. Hence, if the alignment direction is controlled, light can be transmitted or blocked according to the alignment direction of liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, and colors and images can be displayed using color filters.
In an active matrix LCD, an active element is included with each of the pixels arranged in a matrix configuration. Each pixel is controlled using the switching characteristic of this active element. The active matrix LCD is an apparatus that realizes a memory function using electro-optic effects of liquid crystal. However, the active matrix liquid crystal display has a drawback in that its contrast ratio changes depending on the viewing angle.
To resolve the problem of a changing contrast ratio as the viewing angle changes, various methods have been suggested. For example, multidomain technology has been suggested, in which one pixel is divided into several regions (i.e., multidomains) and the alignment of liquid crystal molecules is different depending on the domains. This allow the characteristic of the pixel to be an average of characteristics realized by the domains of one pixel. In another method, phase compensation technology has been suggested in which a phase difference film reduces a phase difference change depending on the viewing angle. Further, In Plane Switching (IPS) mode also has been suggested in which a lateral electric field is applied to the liquid crystal and the liquid crystal direction is twisted on the plane parallel with the alignment film. Furthermore, a vertical alignment mode has been suggested in which liquid crystal with a vertical alignment film and a negative dielectric anisotropy is used. As described above, the solutions to solve the viewing angle problems have been addressed by changing liquid crystal modes used in LCDs.
Generally, the LCD is a passive display device that does not have its own light source. Accordingly, an LCD needs a backlight assembly including a lamp, a light guide and a few sheets, such as polarizer and a diffuser, for a light source. The backlight assembly plays an important role in the display performance of an LCD.
FIG. 1 is an exploded perspective view of a related art LCD module. As shown in FIG. 1, the backlight assembly includes a lamp 1 for emitting light, a reflector 2 for reflecting the emitted light upwards and a light guide 3 for propagating the light emitted from the lamp 1. The backlight assembly further includes diffusers and prisms 4 for diffusing the light that is transmitted upwards and for enhancing light efficiency of the LCD. The backlight assembly is further provided with a lamp housing 5 for reflecting the light emitted from the lamp toward the light guide and for supporting the lamp.
FIG. 2 is a cross sectional view of an optical diffusion sheet employed in a related art backlight assembly. As shown in FIG. 2, the optical diffusion sheet consists of a plurality of layers. More particularly, the optical diffusion sheet is fabricated to enhance the transmissivity of light and to diffuse the light by changing the materials of the films constituting each layer and controlling both an interval and arrangement of beads.
In the related art, correction of the viewing angle problem is attempted mainly by changing the liquid crystal mode and configuring the light diffusion sheet such that transmissivity and luminance are concentrated at the central point of the LCD panel. Thus, the improving transmissivity and display quality through a thickness control of the optical diffusion sheet is a main issue. Further, because it is thought that only the transmissivity of the optical diffusion sheet determines the luminance in the final LCD module state, a lot of research has occurred to enhance the transmissivity of the optical diffusion sheet. Much research also has occurred to enhance the haze of the optical diffusion sheet to correct the viewing angle problem.
FIG. 3 illustrates haze and transmissivity characteristics of a plurality of related art optical diffusion sheets represented as dots on a graph. As shown in FIG. 3, optical diffusion sheets are one of two groups. In other words, an optical diffusion sheet is either a high haze sheet or a high transmissivity sheet. Thus, grouping a high haze sheet together with a high transmissivity can have the effect of reducing the overall luminance.